Pleistocene Glaciers and Geography

Northern Hemisphere Ice Sheets

Ice caps in North America and Europe are well documented. The ice removed
water from the sea, lowering sea level by up to 100 meters. Large areas of the
continental shelves were exposed. Most of the ice north of Siberia accumulated
on what is now the shelf and the extent of Siberian ice is poorly known.
Lowering of sea level also exposed the Bering Strait land bridge.

Glacial Deposits and Landforms in the Continental United States

Retreat of the Ice from North America

Color

Age

White

Present

Magenta (north)

5000-6000

Blue-Gray

6000-7000

Blue

7000-8000

Dark Gray

8000-9000

Dark Green

9000-10,000

Light Green

10,000-11,000

Yellow

11,000-12,000

Light Brown

12,000-13,000

Dark Brown

13,000-15,000

Red

15,000-17,000

Magenta (south)

17,000-18,000

Ice Diversion of Rivers in North America

Ice Diversion of the Missouri River

Top: The Missouri River did not exist in its present form
before the Pleistocene. At that time, the largest drainage in North America flowed
northeast to Hudson's Bay. This map shows the buried pre-Pleistocene valleys in North
Dakota, identified by well drilling.

Middle: During the Pleistocene,
about 2/3 of North Dakota was ice-covered. Water flowed along the ice margins to the
south. This is called an ice-marginal drainage.

Bottom: The ice-marginal drainage remains as the present Missouri
River. Many of the other present rivers of North Dakota are also ice-marginal drainages.
Only a few of the pre-Pleistocene valleys were re-occupied after the ice retreated - the
rest were filled in and are known only from well drilling.

Formation of the Ohio River

The Ohio River did not exist as such before the last ice advances. A great
buried river valley, called the Teays River, probably was the principal east-west drainage
in the Midwest before then. The present Ohio formed when ice blocked the Teays valley and
river headwaters flooded and linked across drainage divides.

The Teays drainage basin is
shown in purple. The Saint Lawrence drainage basin is shown in green.

Pluvial Lakes

During the Pleistocene, a wetter climate in the West caused large lakes to
fill some of the interior basins. The most important ones are:

B indicates Lake Bonneville, which was over 300 meters deep and left
prominent shorelines over much of Utah. The lake filled until it drained north through Red
Rock Pass into the Snake River. The drainage divide is underlain by soft alluvial fan
material, so when the lake cut the outlet, the lake level dropped about 30 meters in a
catastrophic flood.

L denotes Lake Lahontan

M is Lake Manly. Although small, it was fairly deep. It is of interest
because it filled the present Death Valley.

Evolution of the Great Lakes

The English Channel

Before the Pleistocene, the British Isles were probably connected to the
mainland. Submerged valleys hint at a former continuation of the Rhine out onto
the North Sea shelf, with the Thames and probably the Elbe as tributaries.

At ice maximum, only the far south of the British Isles was ice-free. The
Rhine, blocked to the north, flooded its valley and found a new outlet.

Following the retreat of the ice, the sea floods the new valley, creating the
English Channel. Think of all the history that flows from Britain being an
island - the defeat of the Spanish Armada, the opposition to Napoleon and Hitler
- and none of it might have happened if the ice hadn't blocked the North Sea.

Siberian Super-Rivers?

If the ice north of Siberia blocked the major rivers Ob and Yenisey, they
would have ponded until they found new outlets. Lake Baikal might have drained
to the Mediterranean through the longest river in the world - some 6,000 miles.
Later, as the ice retreated, water would have drained along the ice margin to
the Baltic as shown in purple.

Some geologists, however, believe the Siberian ice cap was much smaller and
may not have connected to the Scandinavian ice cap.

Coming to America

The most likely route for humans to enter America was
overland via the Bering land bridge. Some anthropologists believe humans
could not have gotten further south until the clearing of the Ice-Free
Corridor about 12,000 years ago. However, the Corridor was never
extensively blocked by ice, and short ice blockages might not have been a
serious obstacle.

Other anthropologists have suggested that immigrants could have
traveled via the coast, hopping from one ice-free enclave to the next.

Recently, even a trans-Atlantic route has been proposed, perhaps
following the margins of pack ice in pursuit of seals. However, given the
distances involved, that seems like a long shot.

What about much earlier arrivals? It takes time for any species to
expand its range, and there's just no evidence that humans got to Siberia
or the Americas before at most 20,000 years ago.

Given the time scale above (which some anthropologists reject as too old),
humans could have reached Tierra del Fuego at a rate of only a kilometer or so a
year. Even if humans didn't get into the U.S. until 12,000 years ago, they could
have reached the tip of South America migrating only 5-6 kilometers a year.

Who killed the mammoths? The jury is still out. Megafaunas everywhere went
extinct about the time humans arrived, but why hunt mammoths when buffalo are
more abundant? The last mammoths, by the way, lived on Wrangel Island off
Siberia and actually survived until about the time the Pyramids were being
built.